This invention relates to conducting and studying catalysis and catalyzed chemical reactions particularly heterogenesis catalytic reactions.
Development of new catalysts in catalyzed reactions has been hampered by the difficulty encountered in obtaining basic information about the physical and chemical processes involved in catalytic activity and catalytic reactions, such as reaction intermediates, reaction mechanisms, adsorption, and desorption of reactants and products in catalytic reactions, oxidation and reduction of catalysts, catalyst poisons, the concentration of reactants on a catalyst surface, and others.
Classically, this kind of basic information about the chemical and physical processes of catalysis has been deduced primarily from analysis of the final product of the reaction. Conclusions have been based on final products because of the difficulty in isolating and analyzing reaction intermediates, many of which are highly fragile in reactive species. Being able to determine directly the identity of these intermediates and to follow their production and consumption during the reaction would increase the understanding of catalysis and would facilitate the development of catalysts and catalytic processes.
One method that has been used to study the interaction of catalytic surfaces with reactant molecules is called molecular beam mass spectrometry. In this technique, a stream of molecules of reactant gas (a molecular beam) is directed at a target of the catalytic material, with the target oriented at an angle to the molecular beam. The molecules of the reactant gas strike the target, some of them react to form products and intermediates, and they rebound off the target in the direction of an aperture. A portion of the rebounding molecules pass through the aperture into the ionization chamber of a mass spectrometer, which analyzes the mixture for reactants, intermediates, and products.
A variation on this molecular beam technique is called modulated molecular beam mass spectrometry, in which the initial molecular beam of reactant gas is modulated, such as with a rotating "chopper", to produce a series of pulses of the reactant gas. The result is that a series of pulses of gas enter the mass spectrometer for analysis.
In these molecular beam techniques, the entire assembly is enclosed and is operated in a vacuum. The vacuum is necessary to achieve the molecular flow to form the molecular beam, and is necessary for operation of the mass spectrometer.
The vacuum required, along with the fact that the molecules strike the catalyst target and rebound to the detector combine to make the number of reaction opportunities for each molecule of reactant very small. It has been estimated that the number of collisions between a given molecule of reactant gas and the target catalyst would be 10 or less, and that the number of collisions between a given molecule of reactant gas and other gas molecules would also be 10 or less. This means that these molecular beam techniques are practical only for highly reactive systems, in which sufficient reaction occurs in the small number of reaction opportunities to produce detectable amounts of products and intermediates. Most commercially important catalyzed reaction systems are not reactive enough for use with molecular beam techniques. The catalyst suitable for use with molecular beam techniques must be made into a target with a surface regular enough so that the direction of rebound of the reactant gas molecules can be directed toward the mass spectrometer. Not all catalysts can be formed into such a target.
Conventional techniques have been adapted to try to isolate and analyze for reaction intermediates. One common technique involves a reactor containing a catalyst, through which an inert carrier gas flows continuously. A pulse of reactant gas is injected into the carrier gas and is carried through the catalyst. As the product gas exits the reactor, samples are taken and analyzed. This type of system is normally operated at or near atmospheric pressure. The number of collisions between an average molecule of reactant gas and the catalyst is very high, and has been estimated to be far greater than 10.sup.6. Similarly, the number of collisions between an average molecule of reactant gas and other gas molecules has been estimated to be far greater than 10.sup.6. Due to the large number of reaction opportunities, the number of fragile and highly reactive intermediates that emerge from the catalyst is very small, and is usually too small to be detected.
U.S. Pat. Nos. 4,626,412 and 5,009,849 describe a system and process for temporal analysis of the products of catalyzed chemical reactions. The temporal analysis of product system ("TAPS") described in these patents comprises an enclosed housing containing a catalytic reactor, a pulse generator for introducing a pulse of reactant gas into the reactor and for withdrawing a pulse of product gas from the reactor, a collimating slit for producing a resolved pulse of product gas in which the molecules of the product gas move in substantially parallel paths, and a quadrupole for mass spectrometry or other means for providing real time analysis of the resolved pulse product gas. A clock senses the pulse of reactant gas introduced into the reactor by a pulse generator and activates a signal averager that is in communication with the mass spectrometer to receive a signal for a designated period of time and store it. The signal averager stores the signals from a series of pulses and averages them to reduce noise.
The catalytic reactor of the '412 and '849 patents typically contains a particulate catalyst in a physical form comparable to that which would be used in a commercial catalytic reactor. The amount and surface area of the catalyst is sufficient for conversion of reactants to intermediates and products, even in cases where the reaction rate is relatively slow. The collimating slit establishes a beam of reaction product from the product pulse exiting the reactor which is then analyzed in real time by the mass spectrometer.
The TAPS system disclosed in the aforesaid '412 and '849 is operated under high vacuum. Both the reactor and product gas analytical detector are contained within a high vacuum chamber. This chamber is divided into three compartments, the first of which contains the reactor and collimating slit, the second contains a cryogenic trap, and the third contains the quadrupole. A separate vacuum pump is provided for each of the three compartments. In the system of the '412 and '849 patents, operation in both the analytic detection region and the catalytic reactor is conducted under high vacuum.
U.S. Pat. No. 5,039,489 also describes an apparatus for catalyst analysis in which both the catalytic reactor and quadrupole detector of a mass spectrometer are contained within a single vacuum chamber. Alternatively, the '489 patent describes operating the reactor under pressure while the product gas is divided between a stream that is reduced in pressure and subjected to mass spectrometric analysis and another stream which is analyzed on a gas chromatograph. Pressure in the reactor is controlled by a back pressure regulator in parallel with the gas chromatograph. Product gas flowing from the reaction chamber into the vacuum chamber for mass spectrometric analysis passes through a fine orifice in which the pressure of the reaction gas is reduced from the pressure maintained in the reactor to the high vacuum required for analysis of the gas product.
Since prior art methods which apply surface science techniques to the study of catalysts and catalytic reactions have essentially all required that analytical observations be made under very high vacuums, a problem is presented in extrapolating the data to predict effects at the operating pressures of commercial catalytic reactions, nearly all of which are conducted at pressures above 100 torr. The pressure difference, which extends over a number of orders of magnitude, typically has effects on reaction kinetics, absorption and desorption phenomena and the like which compromise the value of observations taken at very high vacuum. This problem, commonly referred to in the literature as the "pressure gap," has inhibited progress in understanding catalytic reactions, and consequently the development of technology for such reactions.